An apparatus for accurate measurement of the acoustic impedance of sound-absorbing materials

This paper describes the principles and the construction of an apparatus for the precise measurement of the acoustic impedance, at normal incidence, of sound-absorbing materials by a stationary-wave method for frequencies in the range 100 to 5000 c/s. From measurements made on a sample of material 1fraction three-quarters inches in diameter, backed by a substantially rigid wall, the magnitudes of the resistive and reactive components of the impedance may be calculated with an accuracy of about one per cent. The theory of the method is discussed with particular reference to the influence thereon of the attenuation of sound associated with the walls of the tube in which the standing wave is formed, and it is shown that in addition to the correction which must be provided to the elementary expression for the " standing-wave ratio ", the finite attenuation in the tube leads to an additional correction to the expression for the distance from the sample of the first minimum of pressure. Attention is drawn to convenient methods which facilitate calculation of the results, and typical results of measurements of the impedance of a sample of porous sound-absorbing material are shown.